The present disclosure generally relates to systems and methods for generating and distributing electrical power, and more particularly such systems and methods which involve multiple electrical generator units connected in parallel.
Typically, a generator is a rotary electric machine of well-known type having a stator surrounded by a rotor that is driven through a belt or shaft by a prime mover (e.g., an engine) to electromagnetically induce electrical current in conductive windings of the stator, whereby mechanical power is converted into electrical power. The stator includes phase coils coupled in a Delta or Wye configuration. A generator may be a DC type that produces direct current or an AC type that produces an alternating current, the latter type also referred to as an alternator. Where used to charge a battery that powers an electrical system, alternator output is rectified. The stator is electrically coupled to the rectifier, which delivers the alternator output to the system bus. A DC generator may include an inverter to convert DC generator output to AC system output power as necessary. Reference herein to a “generator” may refer to either type (i.e., DC or AC) unless one particular type is specified. Often, each generator unit has its own dedicated digital microcontroller (referred to herein as a generator controller) which may be a plug and play device. Each of the generator controllers controls the operation of its respective individual generator unit, and cooperates in the operation of the overall parallel system, which is controlled by a system controller. The generators may coordinate among themselves or designate a system controller that is either internal to one generator or an external electronic control unit.
Parallel generator systems, wherein multiple generators of one type (i.e., DC or AC) are electrically connected to each other in parallel, may be adapted for use in stationary installations, usually to provide backup power for a building or campus, or in mobile installations, and may be a primary power source for charging batteries that provide electrical power for various types of vehicles, such as over-the-road tractors or large buses, for example. Parallel generator systems are well-known for ensuring an uninterrupted supply of power and have significant advantages over single large generator units in areas of cost effectiveness, flexibility, expandability, ease of maintenance and serviceability, and reliability.
The individual generator units operating in parallel systems are typically of smaller capacities, and may be identical or of variable output. In either case, these units can be connected in parallel with paralleling switchgear to achieve maximum output during peak requirement or the desired minimal output during other times.
Using multiple generator units in parallel offers greater flexibility than using a single large-sized generator of a high capacity. Multiple smaller generators operating in parallel do not need to be grouped together and can be distributed such that they are remotely located from each other and do not require a single, large space, as would be needed in the case of a single, larger generator. Furthermore, it is often difficult when sizing generators to match load requirements to accurately project increases in load and adequately plan for anticipated additional loads. By operating generators in parallel, however, variations in load can be relatively easy to accommodate by adding additional parallel-connected generators for additional power supply provided. Thus, by operating generators in a parallel system, it is easier to allow for an increase in the load requirement. Moreover, if a generator unit in the parallel system breaks down or requires maintenance, that individual unit can be removed from service, and repaired or replaced, without disrupting the functioning of the other generator units in the system.
The redundancy inherent in parallel operation of multiple generators provides greater reliability than is offered by single generator unit for critical loads. If one unit fails, the critical loads are redistributed among other units in the system. In many applications, critical loads needing the highest degree of reliable power account for only a fraction of the overall power generated by the system, and parallel systems provide the redundancy necessary to maintain power to critical loads even if one of its generator units fails. The redundancy inherent in a parallel system thus provides multiple layers of protection and ensures an uninterrupted supply of power for critical circuits.
In a parallel generator system, the entire load is shared by all of the parallel-connected generator units operating in the system, the active generator units, and in prior systems load sharing between the active generator units is typically done to ensure all active generator units contribute the same power toward the system load, or so that they all share the same voltage setpoint.
Some parallel generator systems employ a plurality of prime movers to drive the multiplicity of generators. For example, an engine may be dedicated to driving only a respective one of the multiplicity of parallel-connected generators, as is typical in large stationary backup power systems.
Other parallel generator systems, particularly those used in vehicles, employ a single engine to drive the multiplicity of generators. For example, the single engine of an over-the-road tractor or large bus drives each of the multiplicity of parallel-connected generators, which are typically alternators mounted to the engine and driven by the crankshaft through a common belt. Such vehicle-based systems of parallel-connected alternators typically provide rectified DC power to a battery (or multiple batteries) that provides power to the vehicle's electrical system. The multiple alternators may be identical to each other, and may be driven at a common speed that is a ratio of the engine crankshaft speed. The output of the stator windings of each alternator providing power to the system is normally controlled by a single voltage regulator common to all alternators in the system, or a single, dedicated voltage regulator for that respective alternator. The strength of each rotor's moving magnetic field, which induces current flow in the stator windings of the surrounding stator to generate alternator output voltage, is controlled by the voltage regulator(s).
The alternator regulator is configured to control the excitation current to a field coil carried by the rotor and that receives a signal from the regulator having a predetermined duty cycle. The regulator includes a field driver circuit configured to deliver an electric current signal to the field coil at a switching frequency. The field driver circuit is controlled by a dedicated generator controller and is configured to control the field current provided to the field coil. The field driver circuit may include a MOSFET transistor configured to control the electric current delivered to the field coil. The MOSFET transistor is switchable between an on-state and an off-state at the switching frequency. Transistor switching is controlled by the dedicated generator controller. The stator generates an output current having a magnitude that is based on the duty cycle of the signal applied to the field coil.
In general, when more current is provided to the field coil, the output voltage of the alternator increases. When less current is provided to the field coil, the output voltage of the alternator decreases. Vehicle alternators have traditionally utilized fixed frequency field drivers. In these alternators, a field driver circuit provides pulses of voltage to the field coil at a fixed frequency to control current, although some alternators utilize variable frequency field drivers that provide voltage pulses to their field coils at varying frequencies. In particular, the generator controller controls the field current output through the regulator by delivering control signals to the gate of the transistor. These control signals switch the transistor on and off such that the field voltage is provided as a pulse signal. The field voltage signal has a pulse duration τ, and a pulse period T at which the pulses repeat. The duty cycle D is calculated as τ/T. Depending on the inputs received, the dedicated generator controller may adjust the duty cycle D in an attempt to control alternator output by increasing or decreasing the pulse duration τ. The commanded duty cycle of each alternator, which can range between zero and 100 percent, corresponds to the output voltage setpoint.
Generally higher rotor field duty cycles and rotor excitation currents result in higher output current from the generator and higher operating temperatures and other causes of generator unit operating stresses.
More efficient utilization of parallel-connected generators, and thus improved reliability and efficiency of a parallel generator system would be facilitated by systems and methods that better distribute the electrical load shared among the active generators in a manner that better equalizes their operating temperatures, the duty cycles of their rotor fields, and/or the excitation currents of their rotors, thereby equalizing the stress distribution amongst the generator units and maximizing system service life.